Why is it colder in space for astronauts than it is on Earth if you are closer to the Sun?

Answer from Stephen Levy

Assistant Professor of Phyics

Research area: BiophysicsInterests/hobbies: Running, reading

The sun transfers energy to objects far away (like us and the earth) by radiation. Radiation is the energy carried by electromagnetic waves. Light, radio waves and x-rays are all examples of electromagnetic waves that have different energy. Another way to transfer energy to an object is by touching it to something hotter. This type of energy transfer is called conduction.

The space between the sun and earth is mostly empty, so the sun can only transfer energy to objects on earth by radiation. If there were a lot of stuff (like the gas in our atmosphere) that extended all the way between the sun and the earth, then the story would be very different.

Now, let’s forget about the earth for a second and just think about how hot an object gets when it is in direct line of sight of the sun. The temperature that object will reach due to the radiation from the sun depends on how much it reflects (white reflects a lot, whereas black absorbs), its size and its distance from the sun. It is true in this example that being closer to the sun means that the object will get hotter. Let’s remember, though, that the sun is really far away compared to typical satellites that orbit the earth. An object on the International Space Station is only closer to the sun by a few millionths of the earth-sun distance, for example, and would only be about that much hotter (still ignoring the earth).

So, why isn’t it hotter in space? Because there is (almost) nothing in space to absorb the energy from the sun. But if you stick any ‘thing’ in space between the earth and the sun (like a thermometer), that thing will get much hotter than the empty space around it and will also get hotter as it gets closer to the sun (as long as it is in direct line of sight of the sun). Space suits help keep astronauts cold when the sun is shining on them and warm when the sun isn’t shining on them — the difference in temperature between the front and back of a suit can be 275 degrees Fahrenheit, according to NASA.

What happens to the temperature in space when you take that thermometer away? Well, there is actually some other stuff that exists in space that I’ve been ignoring. This stuff is electromagnetic radiation left over from near the beginning of the universe. In a manner of speaking, it has a temperature of about minus 455 degrees Fahrenheit, which is typically what we call the temperature of outer space.

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